[Truffle] Document and simplify a few cases dealing with visibility in dispath.

Author: eregon

core/src/main/java/org/jruby/truffle/nodes/cast/ArrayCastNode.java

@@ -27,7 +27,7 @@ public abstract class ArrayCastNode extends RubyNode {
 
     public ArrayCastNode(RubyContext context, SourceSection sourceSection) {
         super(context, sourceSection);
-        toArrayNode = new DispatchHeadNode(context, false, Dispatch.MissingBehavior.RETURN_MISSING);
+        toArrayNode = new DispatchHeadNode(context, Dispatch.MissingBehavior.RETURN_MISSING);
     }
 
     public ArrayCastNode(ArrayCastNode prev) {

core/src/main/java/org/jruby/truffle/nodes/cast/SplatCastNode.java

@@ -37,6 +37,7 @@ public static enum NilBehavior {
     public SplatCastNode(RubyContext context, SourceSection sourceSection, NilBehavior nilBehavior) {
         super(context, sourceSection);
         this.nilBehavior = nilBehavior;
+        // Calling private #to_a is allowed for the *splat operator.
         toA = new DispatchHeadNode(context, true, Dispatch.MissingBehavior.RETURN_MISSING);
     }
 

core/src/main/java/org/jruby/truffle/nodes/core/ArrayNodes.java

@@ -1829,7 +1829,7 @@ public abstract static class InjectNode extends YieldingArrayCoreMethodNode {
 
         public InjectNode(RubyContext context, SourceSection sourceSection) {
             super(context, sourceSection);
-            dispatch = new DispatchHeadNode(context, false, Dispatch.MissingBehavior.CALL_METHOD_MISSING);
+            dispatch = new DispatchHeadNode(context, Dispatch.MissingBehavior.CALL_METHOD_MISSING);
         }
 
         public InjectNode(InjectNode prev) {

core/src/main/java/org/jruby/truffle/nodes/core/KernelNodes.java

@@ -455,6 +455,7 @@ public abstract static class DupNode extends CoreMethodNode {
 
         public DupNode(RubyContext context, SourceSection sourceSection) {
             super(context, sourceSection);
+            // Calls private initialize_dup on the new copy.
             initializeDupNode = new DispatchHeadNode(context, true, Dispatch.MissingBehavior.CALL_METHOD_MISSING);
         }